The present invention relates to digital radio systems, and more specifically, to performing synchronization as part of the processing of a received signal in a spread spectrum radiocommunication system.
Radiocommunication systems involve the transmission of information over an air interface, for example by modulating a carrier frequency with that information. Upon reception, a receiver attempts to accurately extract the information from the received signal by performing an appropriate demodulation technique. However, in order to demodulate a received signal, it is first necessary to synchronize timing between the transmitter and the receiver. Different levels of synchronization may be required depending upon the design of the radiocommunication system. For example, in most systems clocking differences between the transmitter and the receiver create differences in timing at a bit level. Moreover, in some radiocommunication systems, information is transmitted in bursts, sometimes referred to as "frames", which represent chunks of information that are independently detected and decoded. In these types of systems it is also desirable to locate the beginning of a frame, so that information relevant to a particular receiver is isolated and demodulated. Likewise, some systems (e.g., time division multiple access or TDMA systems) further subdivide frames into timeslots to create channels that are time multiplexed with one another. In these systems it is further desirable to synchronize the receiver to the beginning of each timeslot.
Some systems provide channelization using a spread spectrum technique known as code division multiple access (CDMA). In CDMA systems, the information data stream to be transmitted is first coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The bits of the unique spreading code and long PN-sequence are usually referred to as chips. Thus, in CDMA systems, it is also desirable to synchronize the receiver to the chip boundaries, i.e., bit level synchronization.
To further understand the synchronization tasks associated with signal processing in a CDMA radiocommunication system, consider the following example. FIG. 1 illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. In a CDMA system, base station 10 can transmit signals to mobile stations 14 and 15 as a single (composite) signal. The signal directed to mobile station 14 is typically coded with a short code that is orthogonal or mostly orthogonal to a short code that is used to code the signal directed to mobile station 15. These signals are then spread with a second code that is sometimes referred to as a long code, associated with base station 10. The sum of the two coded and spread signals is then transmitted by base station 10.
When mobile station 14 receives the composite signal, mobile station 14 multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station 14 and the signal directed to mobile station 15 is suppressed as interference noise. Similarly, mobile station 15 multiplies the spread signal with the long code and the short code assigned to mobile station 15 to recreate the signal directed to mobile station 15 and the signal directed to mobile station 14 is suppressed as interference noise. The receivers associated with mobile stations 14 and 15 must have acquired the various levels of synchronization to the received signal which were described above, in addition to learning or knowing the applicable long and short codes, in order to implement despreading, demodulation and decoding of the information residing in that signal.
Many different techniques have been developed in order to acquire synchronization at each of the various levels. For timeslot synchronization these techniques typically depend, in large degree, on the frame structure and the manner in which overhead or control information is conveyed to the mobile station. Overhead information is usually provided on one or more broadcast control channels which are transmitted by base stations using known channels to which mobile stations can quickly lock onto and receive the overhead information including, among other things, information used to acquire frame synchronization with that base station. Those skilled in the art will appreciate that many radiocommunication systems have unsynchronized base stations, i.e., base stations which do not share a common timing reference signal. Accordingly, timeslot synchronization is a process which needs to be performed both at start-up and as a mobile station moves from cell to cell.
In one proposed wideband CDMA system, timeslot synchronization is performed by searching for a synchronization field which is broadcast as part of a control channel at a predetermined position (time) relative to a beginning of a frame or timeslot. The synchronization field is a so-called Gold code, which has good correlation properties to enhance detection at the mobile station. Upon detecting the received Gold code, the mobile station will then know the timeslot boundary, enabling it to read other fields and continue, for example, to perform frame and bit synchronization so that it can read information that is transmitted to it by the system.
A matched filter can be used to detect the Gold code at the mobile station during this part of the synchronization process to perform a correlation with the received signal and look for peaks in the correlation. To accomplish this task, the matched filter would need to have a length equal to the length of the synchronization field. Since this particular application employs wideband CDMA techniques, the Gold code is spread to the chip rate, e.g., 256 chips, by multiplying it with a spreading code associated with the broadcast control channel.
Due to inaccuracy associated with the mobile station's local oscillator, the phase of the synchronization symbol changes from the beginning of the symbol to the end of the symbol. This rotation in phase results in a severe signal energy loss at the peaks of the matched filter output which, in turn, may result in correlation peaks being misdetected. For example, assuming an oscillator having an inaccuracy of 10 ppm, the phase rotation experienced during a synchronization symbol (Gold code) is 450 degrees as calculated by:
Phase Rotation=360.degree..times.Oscillator Inaccuracy.times.Carrier Frequency/ Symbol Rate
where the carrier frequency and symbol rate are assumed to be 2 Ghz and 16 kHz, respectively, for this example. To avoid the signal energy loss associated with this magnitude of phase rotation, it is possible to segment the matched filter into a number of filter segments. This reduces the signal energy loss, but creates a reduction in the signal-to-noise ratio of the received signal. Thus, it would be preferable to find some other technique for performing primary (timeslot) synchronization to a broadcast control channel of a radiocommunication system which avoids the need for a matched filter.